The invention relates to bicycles and systems of transmitting and managing motive power from a rider and electric motor to the bicycle, specifically semi-automatic systems.
Prior art examples of bicycles with automatic shifting systems typically use variables such as bicycle speed and pedaling cadence as shifting criteria. However, it is disadvantageous for a system with a broad gear range and a large number of independent gear ratios to shift entirely automatically. With respect to broad gear ranges, unlike an automatic transmission in a car, it may be undesirable and/or unnecessary for the system to shift to the lowest gear every time the bicycle comes to a full stop.
For example, if a rider riding downhill on a bicycle supplied with an automatic shifting system comes to a complete stop, it might be preferable to shift down only a few gears in preparation for accelerating the bicycle back up to normal riding speed after the stop. In other words, it is not always necessary to start forward motion in a gear that is intended to propel the rider and bicycle up steep hills, for example.
In addition, when shifting down during deceleration and shifting up during acceleration, it may be advantageous to reduce the gear ratio by a small amount, for example only about 40% of the total range of gearing available. Furthermore, in the case where the bicycle has many closely-spaced gears, it does not make sense to shift automatically through each of the gears within a range of 40% each time the bicycle starts and stops. It may be preferable to shift only one or two gears to reach the target gear because the amount of time spent in each gear would be small and the energy used to shift through all of the intermediate gears would result in a lower battery life.
Electric motor-driven bicycles include an electric motor, which is used to assist the rider in propelling the bicycle. Typically, electric motor-driven systems can be described as either direct-drive or geared through a transmission. With respect to the direct-drive version, the motor may be located in the front or rear wheel. When energized, the motor may drive either wheel either directly or through a dedicated speed-reducing (i.e., torque-increasing) transmission—usually a set of gears. Alternatively, the motor may be located elsewhere on the bicycle and a dedicated belt or chain is used to drive either wheel directly. In all these direct-drive cases, the rider and the motor both propel the bicycle in parallel, but only the rider-produced force works through the bicycle transmission. For example, when climbing a hill, the rider may choose a lower gear of the bicycle so that the rider's input at the pedal results in higher torque at the rear wheel. The electric motor by contrast, tries to maintain speed by increasing torque and drawing more current from an attached battery.
A second class of, electric bicycle transmission is characterized by a motor driving the bicycle through a bicycle transmission. This is commonly referred to as a mid-ship gearbox. In this transmission, the gear that the rider chooses to pedal through is also the gear the motor loads pass through, both driving the rear wheel. The motor does this by directly driving the front chainring in parallel to the rider driving the front chainring. The motor may drive the chainring directly or through a dedicated speed-reducing transmission that slows down the motor speed while increasing the torque. The benefit to a speed-reducing transmission for the motor is that the motor and the rider can then pick appropriate gearing that allows both to operate in an efficient range. For example, when the bicycle is accelerating from a stop, the motor first delivers high torque at the wheel and accelerates quickly to a first speed. Once this speed is reached, the gear should be changed to a lower torque/higher speed output until a middle speed is reached and then changed to a lowest torque/highest speed output in order to reach a final target speed.
Electric motors have a speed and load at which they operate most efficiently. However, until a motor is up to its most efficient speed, it produces a high torque to reach that speed and runs very inefficiently. It would be beneficial to allow the motor to run at its most efficient speed during acceleration.
FIGS. 13 and 14 respectively show a direct-drive electric bicycle motor accelerating to a desired, highly efficient speed and an electric bicycle motor that shifts through three distinct gears. It can be seen that the system of FIG. 14, which operates at higher torque because of the gearing, allows the motor to reach a more efficient speed before each shift is reached.
FIG. 15 shows the speed of the electric motor and directly compares the speed of the motor of a direct-drive system (with no gears) to a motor and drive system with gearing. The curve on the left, representing the geared system, reaches an efficient speed more quickly and stays in the efficient range for more time during acceleration. It also is clear that the geared system is able to accelerate more rapidly.
One concern regarding a geared, motorized system is that the loads on the bicycle drivetrain are a sum of the loads generated by the rider rotating a crankset plus the load generated by the motor. For example, a rider may generate 200 watts of power and the electric motor may generate 400 watts of power. The drivetrain, generally consisting of the front chainring, chain, rear cassette and hub will then see about 600 watts of power. This amount of power is considered hard on drivetrains. It increases wear rates and tends to increase the load on the drivetrain when the rider is attempting to manually shift gears. For bicycles without an electric motor, when a rider initiates a gear shift, the rider typically reduces the force on the drivetrain by easing pedaling forces to facilitate a smoother and quieter shift. If a rider attempts to shift under full load there is a risk of breaking the system as one gear is disengaged and another gear is engaged. Bicycle drivetrains are designed to function under both scenarios, but it should be understood that higher shift loads promote premature wear and the risk of breakage while riding.
In one existing electric drive system for a bicycle, the motor drives the rear wheel through a geared transmission producing a gear ratio selected by the rider. The system does not automatically choose the correct gear ratio. The electric motor driving the bicycle is not configured to, sense when the rider attempts to make a shift, so the drivetrain sees a full load applied by the motor even during the shift. This can damage the transmission, which can be expensive to replace or result in a derailment, which is unsafe for the rider.
There is a need for a system for propelling a bicycle through a drive system that provides for efficient and safe use of the drive system. The invention satisfies the need.